Process for the conditioning of radioactive iodine, particularly iodine 129, using an apatite as the confinement matrix

ABSTRACT

The invention relates to the conditioning or packaging of radioactive iodine, particularly iodine 129, using an apatite as the confinement matrix. Having the iodine, said apatite corresponds to the formula: 
     
         M.sub.10 (XO.sub.4).sub.6-6x (PO.sub.4).sub.6x I.sub.2     (I) 
    
     in which M represents Cd or Pb, X represents V or As, I is the radioactive iodine to be conditioned and x is such that 0≦x&lt;1. This iodoapatite (1) can be surrounded by an apatite (3) not containing iodine serving as a physical barrier. 
     The iodoapatite can be obtained from a solid compound of the iodine, e.g. an iodide such as silver iodide or lead iodide, by reaction with a compound of formula: 
     
         M.sub.3 (XO.sub.4).sub.2-2x (PO.sub.4).sub.2x              (II) 
    
     or 
     
         M.sub.10 (XO.sub.4).sub.6-6x (PO.sub.4).sub.6x Y.sub.2     (III) 
    
     in which M, X and x are as defined hereinbefore and Y can represent OH, F, Cl or O 1/2 .

The present invention relates to the conditioning or packaging ofradioactive iodine, particularly iodine 129, which is a β and γ emittingfission product having a decay period of 1.6.10⁷ years.

Radioactive iodine is present in irradiated fuels from nuclear reactors.This iodine is released when said fuels are reprocessed. Thus, gaseousiodine occurs in the gases emitted by the irradiated fuel dissolvingsolution and iodine traces appear in aqueous effluents. As iodine 129 istoxic for humans due to its strong affinity for the thyroid gland, it isnecessary to eliminate said iodine and store it on a definitive basisfor a long time due to its very high period, although the specificradioactivity of iodine 129 is very low, because a high iodine 129concentration would be dangerous to health. It is therefore vital tocondition and store iodine 129 in a reliable matrix.

Existing methods for the trapping of iodine 129 lead to the obtaining ofsilver iodide, copper iodide, lead iodide or barium iodate. For storingthe thus trapped iodine, several procedures have been studied andconsideration has been given to the storage thereof in ceramic phases orin low melting point glasses, but a stable phase is still being soughtfor long term storage purposes.

The present invention relates to a block for the conditioning ofradioactive iodine, particularly iodine 129, which uses as theconfinement matrix a material having properties particularly appropriatefor long term storage.

According to the invention, the radioactive iodine conditioning blockcomprises an iodoapatite of formula:

    M.sub.10 (XO.sub.4).sub.6-6x (PO.sub.4).sub.6x I.sub.2     (I)

in which M represents Cd or Pb, X represents V or As, I is theradioactive iodine to be conditioned and x is such that 0≦x<1.

In this block, the iodine is chemically trapped in an apatite structure,which has very advantageous properties for a long term conditioning.

Thus, apatites have the very interesting property of being able tointegrate into their structure other elements and in particulardifferent halogens such as iodine. Moreover, apatites have the followingremarkable properties:

their structure is highly chemically and thermally stable,

apatites have a very limited solubility in water and their solubilitydecreases when the temperature increases,

apatite structures are able to withstand β and γ radioactivity and

apatites can receive in their lattice molecular species such as oxygen,so that they are able to receive the non-radioactive xenon produced bythe radioactive disintegration of iodine 129 without embrittlement orincreasing the porosity of the conditioning matrix.

Natural fluoapatite complies with the following formula:

    Ca.sub.10 (PO.sub.4).sub.6 F.sub.2

In this structure, numerous substitutions can be made and in particularthe calcium can be replaced by various divalent cations such as cadmium,strontium, barium, lead, etc., the phosphate ions can be substituted byvanadate or arsenate ions and the F⁻ anions can be substituted bymonovalent anions such as I⁻. Due to the size of the I⁻ anion, it isonly possible to replace F⁻ by I⁻ in apatites complying with generalformula I in which M is Cd or Pb, X is V or As and 0≦x<1.

Thus, in the block according to the invention, the replacement of thephosphate groups of natural apatite by more voluminous VO₄ or AsO₄groups leads to a significant increase in the lattice constants. Thisleads to an increase in the section of the tunnels of the apatite,because said section is directly linked with the value of the latticeconstant a and this makes it possible to introduce into the tunnels aniodide ion, whose ion radius (2.20 Å) is much larger than that of the F⁻or Cl⁻ ions (1.33 and 1.81 Å respectively) present in the naturalapatite.

In the same way, the substitution of the Ca²⁺ cation of the naturalapatite by a more voluminous cation such as Pb, leads to an increase inthe lattice constants and facilitates the introduction of I⁻ into thetunnels.

In the case of Cd²⁺, which has an ion radius (0.95 Å) smaller than thatof Ca²⁺ (1.00 Å), the possibility exists to introduce I⁻ in place of F⁻or Cl⁻, which may be explainable by the strong polarizability of theCd²⁺ ion and also the presence of XO₄ ³⁻ ions, which are more voluminousthan PO₄ ³⁻.

As will be shown hereinafter, the block according to the invention canbe prepared by reacting an iodine-containing compound with a solidcompound of formula:

    M.sub.3 (XO.sub.4).sub.2-2x (PO.sub.4).sub.2x              (II)

in which M, X and x have the meanings given hereinbefore.

According to the invention, it may be advantageous not to totallyreplace the PO₄ ³⁻ ions of the natural apatite by VO₄ ³⁻ or AsO₄ ³ ⁻ions, because it is preferable that the solid compound (II), in the casewhere M=Pb, X=V and x=0, is in the γ phase in the useful temperaturerange for the production of the block, namely 20° to 800° C.

However, it is known that in cases where x=0, M is Pb and X representsV, the solid compound, lead orthovanadate, undergoes a β-γ phasetransition at 120° C., which induces a 1.4% volume contractionprejudicial to the long term good behaviour of the material, i.e. theconditioning block of the invention.

However, when the VO₄ ³⁻ ions are partly replaced by PO₄ ³⁻ ions (x>0)the phase transition temperature is lowered, it e.g. appearing at -50°C. when x=0.2. Thus, for x=0.2, the material undergoes no phasetransition the temperature range used for producing the block accordingto the invention. It is consequently of interest to retain part of thePO₄ ³⁻ ions in order to prevent an embrittlement of the block during itsproduction.

Preferably, x is such that 0.1≦O≦0.75 and good results are obtained forx between 0.1 and 0.3.

According to the invention, it is possible to further improve theperformance characteristics of the conditioning by surrounding theiodoapatite containing in its structure the radioactive iodine to beconditioned, by one or more layers of apatites not containing iodinehaving various compositions and serving as a physical barrier resistingexternal attacks and stresses.

The composition of the different layers can be modified in such a waythat the internal layer or layers ensure the trapping of the iodine,whereas the external layer or layers resist attacks from the externalmedium.

The apatites not containing iodine used are chosen as a function oftheir properties, so that the conditioning has both a good resistance todissolving in water and a good resistance to irradiation damage. As anexample of a usable apatite, reference is made to phosphocalciumfluoapatites and phosphosilicate fluoapatites (britholites).

In order to chemically trap iodine in an apatite structure iniodoapatite form, it is possible to start with an iodine-containingcompound in the solid state, such as a metal iodide, and react it with acompound of formula:

    M.sub.3 (XO.sub.4).sub.2-2x (PO.sub.4).sub.2x              (II)

in which M, X and x have the meanings given hereinbefore, also in thesolid state, at a temperature between 500° and 800° C.

This solid/solid reaction corresponds to the following diagrams, in thecases where the starting iodine-containing compound is PbI₂ or AgI:

    PbI.sub.2 +3 M.sub.3 (XO.sub.4).sub.2-2x (PO.sub.4).sub.2x !→PbM.sub.9 (XO.sub.4).sub.6-6x (PO.sub.4).sub.6x I.sub.2

    AgI+3 M.sub.3 (XO.sub.4).sub.2-2x (PO.sub.4).sub.2x !→AgM.sub.9 (XO.sub.4).sub.6-6x (PO.sub.4).sub.6x I □

the symbol □ representing a lacuna in the iodine site.

This reaction can take place on the basis of fine powders of iodide andthe compound of formula (II), by subjecting them to sintering at 500° to800° C. The sintering time is chosen as a function of the temperatureused and can range between 1 and 3 hours. This reaction is preferablyperformed on a mixture of powders compressed under an isostatic oruniaxial pressure of e.g. 50 to 200 MPa (5 to 20 kbar). The mixture canbe compressed in moulds having the shape of blocks or pellets.

The use of pressure during sintering permits a more intimate contactbetween the powders and a better confinement of the iodine during theconsolidation of the mixture in the form of blocks or pellets, whichconsequently have good mechanical properties with a view to a long termstorage.

The compounds of formula M₃ (XO₄)_(2-2x) (PO₄)_(2x) can be prepared byconventional processes.

In the case where M represents Pb and x=0, it is possible to obtainthese compounds by the solid/solid reaction of a mixture of lead oxideor vanadium pentoxide or lead oxide and NH₄ H₂ AsO₄ or As₂ O₅.nH₂ O, ata temperature of approximately 700° C.

In the case where M represents Cd, a process of a similar nature can beused and the lead oxide is replaced by cadmium oxide.

According to a variant of the invention, when the radioactive iodine isin the gaseous state or in the form of a sublimatable, iodine-containingcompound, it is possible to obtain the iodoapatite trapping theradioactive iodine of formula (I) from an apatite of formula:

    M.sub.10 (XO.sub.4).sub.6-6x (PO.sub.4).sub.6x Y.sub.2     (III)

in which M, X and x have the meanings given hereinbefore and Yrepresents F, Cl, OH or O_(1/2), by contacting said apatite with a gascontaining gaseous iodine or the sublimatable compound vapour, in orderto exchange Y by radioactive iodine and fix the iodine iniodine-containing apatite form.

The starting apatite of formula (III) can be prepared by conventionalprocesses, e.g. by the double decomposition of lead nitrate and vanadiumpentoxide, in an aqueous medium, in the case where M represents lead, Xrepresents V, Y represents OH and x=0.

According to the invention, the radioactive iodine conditioning blockcan be produced so as to incorporate, as from the start of the long termstorage, the radioactive iodine in the form of the iodoapatite offormula (I). However, it is also possible to produce it from differentconstituents, whereof one contains the radioactive iodine in the form ofa solid iodine-containing compound, by carefully distributing theconstituents within the block in order to form, during the long termstorage, the iodoapatite of formula (I).

In the latter case, according to a first embodiment, the block forconditioning the radioactive iodine in the form of a solid,iodine-containing compound, comprises a core formed from saidiodine-containing compound, surrounded by a first compacted powder layerof a compound complying with one of the formulas:

    M.sub.3 (XO.sub.4).sub.2-2x (PO.sub.4).sub.2x              (II)

or

    M.sub.10 (XO.sub.4).sub.6-6x (PO.sub.4).sub.6x Y.sub.2     (III)

in which M represents Cd or Pb, X represents V or As, Y represents OH,F, Cl or O_(1/2) and x is such that 0≦x<1 and a second outer layer ofapatite not containing iodine.

According to a second embodiment, the block for conditioning theradioactive iodine in the form of a solid, iodine-containing compoundcomprises granules of said iodine-containing compound coated with alayer of a compound complying with one of the formulas:

    M.sub.3 (XO.sub.4).sub.2-2x (PO.sub.4).sub.2x              (II)

or

    M.sub.10 (XO.sub.4).sub.6-6x (PO.sub.4).sub.6x Y.sub.2     (III)

in which M represents Cd or Pb, X represents V or As, Y represents OH,F, Cl or O_(1/2) and x is such that 0≦x<1, the coated granules beingdispersed in a matrix of apatite not containing iodine.

Generally, the iodine-containing compound in the solid state is a metaliodide such as AgI or PbI₂, in the first embodiment.

The iodine-containing compounds used as the starting product forproducing the blocks according to the invention correspond to thecompounds obtained during the elimination of iodine from aqueouseffluents and gaseous effluents of reprocessing plants, or are directlyprepared therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention can be gathered from thefollowing description relative to non-limitative, exemplifiedembodiments and with respect to the attached drawings, wherein show:

FIG. 1 Diagrammatically a conditioning block according to the invention.

FIG. 2 A first embodiment of a conditioning block according to theinvention, in which the iodoapatite fixing the radioactive iodine formsduring the long term storage.

FIG. 3 A second embodiment of a conditioning block according to theinvention, where once again the iodoapatite forms during the long termstorage.

FIG. 1 shows a radioactive iodine conditioning block according to theinvention comprising a core 1 formed from iodoapatite complying withformula (I), surrounded by a layer 3 of apatite not containing iodineand serving as a protective barrier against external attacks andstresses.

The following procedure is used for producing such a block, in which theiodoapatite complies with the formula Pb₁₀ (VO₄)₆ I₂.

Firstly lead orthovanadate of formula Pb₃ (VO₄)₂ is prepared by mixingin stoichiometric proportions a lead oxide powder and a vanadium oxidepowder, both having an average grain size of 20 μm, and by making saidmixture undergo at least two cycles, each involving a heat treatment at700° C. and grinding at ambient temperature spread over a period ofapproximately 6 hours.

Mixing then takes place in stoichiometric proportions of the previouslyobtained lead orthovanadate powder (average grain size 1 μm) and a leadiodide powder (average grain 10 μm) containing the radioactive iodine tobe conditioned. The mixture is then treated at 700° C. for 1 h in astainless steel reactor in order to form the iodoapatite of the core 1.The latter is obtained by compression, during or after iodoapatitesynthesis, under a pressure of at least 1 MPa. The thus obtained part isthen placed in a storage container and is surrounded by a protectivebarrier 3 filling the space between the part and the container. Thisbarrier 3 is constituted by synthetic apatites (fluoapatite orbritholites) or natural apatites.

FIG. 2 shows a first embodiment of a conditioning block according to theinvention, in which the iodoapatite forms during long term storage. Inthis case, the radioactive iodine to be conditioned is in the form of asolid, iodine-containing compound, e.g. lead iodide or silver iodide.This compound forms the core 21 of the block and is surrounded by afirst layer 23 of a compound of formula M₃ (XO₄)_(2-2x) (PO₄)_(2x) orformula M₁₀ (XO₄)_(6-6x) (PO₄)_(6x) Y₂, in which M, X, Y and x have themeanings given hereinbefore, and a second layer 25 of apatite notcontaining iodine constituting a protective apatite matrix. The assemblyconstituted by the core 21 and the layer 23 undergoes sintering underpressure of e.g. 20 to 200 MPa in a furnace, at a temperature of 500° to800° C. and for 1 to 3 h.

The conditioning block can be obtained by compressing (P≧1 MPa) thefritted assembly (21, 23) and the second layer (25) of apatite notcontaining iodine and by subjecting everything to sintering under apressure of e.g. 20 to 200 MPa, in a furnace, at a temperature of 500°to 800° C. and for 1 to 3 h.

FIG. 3 shows another embodiment of a conditioning block according to theinvention, in which the iodoapatite forms during the long term storage.In this case, granules 31 of a solid, iodine-containing compoundcontaining the radioactive iodine to be conditioned are coated with alayer 33 of a compound complying with one of the formulas M₃(XO₄)_(2-2x) (PO₄)_(2x) and M₁₀ (XO₄)_(6-6x) (PO₄)_(6x) Y₂ in which M,X, Y and x have the meanings given hereinbefore, and are dispersed in amatrix of apatite not containing iodine forming a physical barrier.

This block can be prepared in the following way. Firstly the granules ofthe solid, iodine-containing compound, e.g. silver iodide or lead iodideare prepared by a conventional method. These granules 31 are thencovered with a layer 33 of M₃ (XO₄)_(2-2x) (PO₄)_(2x) or M₁₀(XO₄)_(6-6x) (PO₄)_(6x), and the assembly undergoes sintering underpressure, optionally under isostatic pressure, under conditionsidentical to those described in conjunction with 21, 23 of FIG. 2. Theyare then dispersed in a non-iodide-containing apatite powder forming thematrix 35 and everything is subject to a pressurized sintering at 20 to200 MPa under conditions identical to those described for the block ofFIG. 2.

According to a variant of the block according to the invention,applicable in the case of the blocks of FIGS. 2 and 3, the assemblyformed by the iodine-containing compound surrounded by the first layerof M₃ (XO₄)_(2-2x) (PO₄)_(2x) or M₁₀ (XO₄)_(6-6x) (PO₄)_(6x) Y₂ and theouter layer of non-iodine-containing apatite undergoes compression undera pressure of at least 1 MPa and then everything undergoes pressurizedsintering under the same conditions, e.g. pressure 20 to 200 MPa,temperature 500° to 800° C. and duration 1 to 3 h, as hereinbefore.

A description will now be given of the production of a conditioningblock by the synthesis of a weakly PO₄ substituted iodoapatite offormula:

    Pb.sub.10 (VO.sub.4).sub.4.8 (PO.sub.4).sub.1.2 I.sub.2 (x=0.2)

This synthesis corresponds to the reaction:

    3Pb.sub.3 (VO.sub.4).sub.1.6 (PO.sub.4).sub.0.4 +PbI.sub.2 →Pb.sub.10 (VO.sub.4).sub.4.8 (PO.sub.4).sub.1.2 I.sub.2

Preparation takes place of a composite ceramic constituted by a PbI₂core and an enveloping or covering layer of Pb₃ (VO₄)₁.6 (PO₄)₀.4 bysintering at 700° C. under 25 MPa.

This composite ceramic is then covered with a layer of fluoapatite Ca₁₀(PO₄)₆ F₂ and sintering takes place at 700° C., under 25 MPa, to obtaina block having an identical structure to that of the block of FIG. 2. Inthis case reference 21 represents PbI₂, reference 23 represents Pb₃(VO₄)₁.6 (PO₄)₀.4 and reference 25 represents Ca₁₀ (PO₄)₆ F₂.

The blocks obtained according to the invention makes it possible toguarantee an effective, reliable storage of radioactive iodine, such as¹²⁹ I, for very long periods.

We claim:
 1. Block for conditioning radioactive iodine, characterized inthat it comprises an iodoapatite of formula:

    M.sub.10 (XO.sub.4).sub.6-6x (PO.sub.4).sub.6x I.sub.2     (I)

in which M represents Cd or Pb, X represents V or As, I is theradioactive iodine to be conditioned and x is such that 0≦x<1.
 2. Blockaccording to claim 1, characterized in that the iodoapatite containingthe radioactive iodine to be conditioned is surrounded by one or morelayers of apatite not containing iodine.
 3. Block for conditioningradioactive iodine in the form of a solid, iodine-containing compound,characterized in that it comprises a core formed by saidiodine-containing compound, surrounded by a first compacted powder layerof a compound complying with one of the formulas:

    M.sub.3 (XO.sub.4).sub.2-2x (PO.sub.4).sub.2x              (II)

or

    M.sub.10 (XO.sub.4).sub.6-6x (PO.sub.4).sub.6x Y.sub.2     (III)

in which M represents Cd or Pb, X represents V or As, Y represents OH,F, Cl or O_(1/2) and x is such that 0≦x<1, and a second outer layer ofnon-iodine-containing apatite.
 4. Block for conditioning radioactiveiodine in the form of a solid, iodine-containing compound, characterizedin that it comprises granules of said iodine-containing compound coveredwith a layer of a compound complying with one of the formulas:

    M.sub.3 (XO.sub.4).sub.2-2x (PO.sub.4).sub.2x              (II)

or

    M.sub.10 (XO.sub.4).sub.6-6x (PO.sub.4).sub.6x Y.sub.2     (III)

in which M represents Cd or Pb, X represents V or As, Y represents OH,F, Cl or O_(1/2) and x is such that 0≦x<1, the coated granules beingdispersed in a matrix of apatite not containing iodine.
 5. Blockaccording to claim 3, characterized in that the compound of formula (M₃(XO₄)_(2-2x) (PO₄)_(2x) is Pb₃ (VO₄)₂.
 6. Block according to claim 1,characterized in that x is such that 0.1≦x≦0.75.
 7. Block according toclaim 2, characterized in that the apatite not containing iodine ischosen from among phosphocalcium fluoapatites and phosphosilicatefluoapatites.
 8. Block according to claim 3, characterized in that theiodine-containing compound is AgI or PbI₂.
 9. Block according to claim1, characterized in that the radioactive iodine is iodine
 129. 10.Process for the conditioning of the radioactive iodine present in theform of a solid, iodine-containing compound, characterized in that itconsists of reacting the iodine-containing compound with a solidcompound of formula:

    M.sub.3 (XO.sub.4).sub.2-2x (PO.sub.4).sub.2x              (II)

in which M represents Cd or Pb, X represents V or As and x is such that0≦x<1, also in the solid state, at a temperature of 500° to 800° C. 11.Process according to claim 10, characterized in that theiodine-containing compound is AgI or PbI₂.
 12. Process for theproduction of a radioactive iodine conditioning block according to claim3, characterized in that it consists of subjecting to a pressurizedsintering the core of the block and the layer of the compound of formulaM₃ (XO₄)_(2-2x) (PO₄)_(2x) or M₁₀ (XO₄)_(6-6x) (PO₄)_(6x) Y₂,surrounding everything with the non-iodine-containing apatite powderforming the outer layer and subjecting the sintered assembly and theouter layer to a pressurized sintering.
 13. Process for the productionof a radioactive iodine conditioning block according to claim 3,characterized in that it consists of subjecting to a compression under apressure of at least 1 MPa the assembly formed by the iodine-containingcompound, surrounded by the first layer of compound of formula (II) or(III) and the outer layer of non-iodine-containing apatite and thensubjecting everything to pressurized sintering.
 14. Process according toclaim 12, characterized in that sintering is performed at a temperatureof 500° to 800° C., under a pressure of 20 to 200 MPa and for 1 to 3 h.15. Block according to claim 4, characterized in that the compound offormula M₃ (XO₄)_(2-2x) (PO₄)_(2x) is Pb₃ (VO₄)₂.
 16. Block according toclaim 3, characterized in that x is such that 0.1≦x≦0.75.
 17. Blockaccording to claim 4, characterized in that x is such that 0.1≦x≦0.75.18. Block according to claim 3, characterized in that thenon-iodine-containing apatite is chosen from among phosphocalciumfluoapatites and phosphosilicate fluoapatites.
 19. Block according toclaim 4, characterized in that the non-iodine-containing apatite ischosen from among phosphocalcium fluoapatites and phosphosilicatefluoapatites.
 20. Block according to claim 4, characterized in that theiodine-containing compound is AgI or PbI₂.
 21. Block according to claim3, characterized in that the radioactive iodine is iodine
 129. 22. Blockaccording to claim 4, characterized in that the radioactive iodine isiodine
 129. 23. Process for the production of a radioactive iodineconditioning block according to claim 4, characterized in that itconsists of subjecting a pressurized sintering the granulesiodine-containing compound and the layer of the compound of formula M₃(XO₄)_(2-2x) (PO₄)_(2x) or M₁₀ (XO₄)_(6-6x) (PO₄)_(6x) Y₂, surroundingeverything with the non-iodine-containing apatite powder forming theouter layer and subjecting the sintered assembly and the outer layer topressurized sintering.
 24. Process for the production of a radioactiveiodine conditioning block according to claim 4, characterized in that itconsists of subjecting to compression under a pressure of at least 1 MPathe assembly formed by the granules of iodine-containing compoundsurrounded by the first layer of compound of formula (II) or (III) andthe outer, non-iodine-containing apatite layer, then subjectingeverything to pressurized sintering.
 25. Process according to claim 23,characterized in that sintering is performed at a temperature of 500° to800° C. under a pressure of 20 to 200 MPa and for 1 to 3 h.